The invention relates generally to earth-boring bits used to drill a borehole for the ultimate recovery of oil, gas or minerals. More particularly, the invention relates to rolling cone rock bits and to an improved cutting structure for such bits. Still more particularly, the invention relates to enhancements in cutting element orientation so as to improve bit durability and rate of penetration.
An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by revolving the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. With weight applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone. The borehole formed in the drilling process will have a diameter generally equal to the diameter or “gage” of the drill bit.
An earth-boring bit in common use today includes one or more rotatable cutters that perform their cutting function due to the rolling movement of the cutters acting against the formation material. The cutters roll and slide upon the bottom of the borehole as the bit is rotated, the cutters thereby engaging and disintegrating the formation material in its path. The rotatable cutters may be described as generally conical in shape and are therefore sometimes referred to as rolling cones or rolling cone cutters. The borehole is formed as the action of the rotary cones remove chips of formation material that are carried upward and out of the borehole by drilling fluid which is pumped downwardly through the drill pipe and out of the bit.
The earth disintegrating action of the rolling cone cutters is enhanced by providing a plurality of cutting elements on the cutters. Cutting elements are generally of two types: inserts formed of a very hard material, such as tungsten carbide, that are press fit into undersized apertures in the cone surface; or teeth that are milled, cast or otherwise integrally formed from the material of the rolling cone. Bits having tungsten carbide inserts are typically referred to as “TCI” bits or “insert” bits, while those having teeth formed from the cone material are known as “steel tooth bits.” In each instance, the cutting elements on the rotating cutters break up the formation to form the new borehole by a combination of gouging and scraping or chipping and crushing. The shape and positioning of the cutting elements (both steel teeth and tungsten carbide inserts) upon the cone cutters greatly impact bit durability and rate of penetration (ROP) and thus, are important to the success of a particular bit design.
In oil and gas drilling, the cost of drilling a borehole is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed in order to reach the targeted formation. This is the case because each time the bit is changed, the entire string of drill pipes, which may be miles long, must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section.
As is thus obvious, this process, known as a “trip” of the drill string, requires considerable time, effort and expense. Because drilling costs are typically thousands of dollars per hour, it is thus always desirable to employ drill bits which will drill faster and longer, and which are usable over a wider range of formation hardness. The length of time that a drill bit may be employed before it must be changed depends upon its ability to “hold gage” (meaning its ability to maintain a full gage borehole diameter), its rate of penetration (ROP), as well as its durability or ability to maintain an acceptable ROP.
The inserts in TCI bits are typically positioned in circumferential rows on the rolling cone cutters. To assist in maintaining the gage of a borehole, conventional rolling cone bits typically employ a heel row of hard metal inserts on the heel surface of the rolling cone cutters. The heel surface is a generally frustoconical surface and is configured and positioned so as to generally align with and ream the sidewall of the borehole as the bit rotates. The inserts in the heel surface contact the borehole wall with a sliding motion and thus generally may be described as scraping or reaming the borehole sidewall. The heel inserts function primarily to maintain a constant gage and secondarily to prevent the erosion and abrasion of the heel surface of the rolling cone. Excessive wear of the heel inserts leads to an undergage borehole, decreased ROP, increased loading on the other cutting elements on the bit, and may accelerate wear of the cutter bearings, and ultimately lead to bit failure.
Conventional bits also typically include one or more rows of gage cutting elements. Gage cutting elements are mounted adjacent to the heel surface but orientated and sized in such a manner so as to cut the corner of the borehole. In this orientation, the gage cutting elements generally are required to cut both the borehole bottom and sidewall. The lower surface of the gage cutting elements engages the borehole bottom, while the radially outermost surface scrapes the sidewall of the borehole.
Conventional bits also include a number of additional rows of cutting elements that are located on the cones in rows disposed radially inward from the gage row relative to the bit axis. These cutting elements are sized and configured for cutting the bottom of the borehole and are typically described as inner row cutting elements and, as used herein, may also be described as bottomhole cutting elements. Such cutting elements are intended to penetrate and remove formation material by gouging and fracturing formation material. In many applications, inner row cutting elements are relatively longer and sharper than those typically employed in the gage row or the heel row where the inserts ream the sidewall of the borehole via a scraping or shearing action.
Inner row inserts in TCI bits have been provided with various geometries. The cutting surfaces of some inserts have a symmetric geometry, while the cutting surface of other inserts have an asymmetric geometry. For example, a “conical” insert having a cutting surface that tapers from a cylindrical base to a generally rounded or spherical apex is one common symmetric insert. Such an insert is shown, for example, in FIGS. 4A-C in U.S. Pat. No. 6,241,034. Another common symmetric insert is the semi-round top or domed insert having a spherical cutting surface. On the other hand, a “chisel” crested insert is an example of an asymmetric insert. Rather than having the spherical apex of the conical insert, a chisel insert includes two generally flattened sides or flanks that converge and terminate in an elongate crest at the terminal end of the insert. The chisel element may have rather sharp transitions where the flanks intersect the more rounded portions of the cutting surface, as shown, for example, in FIGS. 1-8 in U.S. Pat. No. 5,172,779. Other common asymmetric inserts include a crest or apex that is offset from the insert's axis, or having cutting surfaces that vary in geometry about the circumference of the cutting portion of the insert.
By rotating an asymmetric insert about its central axis when it is mounted to the rolling cone cutter, the geometry of the leading side of the insert may be varied. As used herein, the term “leading” may be used to describe a side, half, or particular region of the cutting surface of an insert that leads the insert relative to a particular direction of motion (e.g., direction of rotation of the cone cutter to which the insert is mounted), whereas the term “trailing” may be used to describe a side, half, or particular region of the cutting surface of an insert that trails or follows the leading side relative to that particular direction of motion. In other words, for a given direction of motion, the leading side of the insert faces the direction of direction of motion and the trailing side faces away from the direction of motion. For a given direction of motion, the trailing side of an insert is generally disposed opposite or 180° from the leading side. Depending on the application and the type of formation to be drilled, it may be preferred to orient an insert such that a particular portion or side of the insert first impacts the formation during drilling. For instance, some asymmetric inserts are designed to include cutting surfaces with particular regions designed and tailored to impact the formation, and break, crush, and shear the formation material, and other regions designed and tailored to trail and support the impacting portion of the insert, and scrape across the newly exposed formation material.
In many conventional rolling cone bits, the inserts on each given cone cutter are mounted in substantially identical orientations relative to the direction of rotation of the cone cutter about the cone axis. In other words, each insert is oriented such that the same region or portion of the cutting surface is disposed on the leading side of the insert relative to the direction of rotation of the cone cutter. This approach generally assumes that all the inserts on a given cone move in the same direction—the direction of rotation of the cone cutter about the cone axis. However, when the rotation of the entire bit about the central axis of the drill string is taken into account along with the rotation of the individual rolling cone cutters about their respective cone axis, inserts mounted in different regions of a given cone cutter actually move in opposite directions relative to each other. In particular, the combined effect of the bit rotation and the cone rotation results in the radially innermost inserts (relative to the bit axis) on a given cone moving in a first direction, whereas the radially outermost inserts (relative to the bit axis) on same cone move in a second direction generally opposite the first direction. With identical and uniform insert orientations in the cone cutter relative to the direction of rotation of the cone about the cone axis, the geometry of the portion of the cutting surface of the radially innermost inserts that leads the insert into the formation during drilling may be different than the geometry of the portion of the cutting surface of the radially outermost inserts that leads the insert into the formation. Those regions of the cutting surface that are not specifically designed or tailored to impact and lead the insert into the formation during drilling may be particularly susceptible to premature chipping, breaking, or damage. Once the cutting structure is damaged and the ROP is reduced to an unacceptable rate, the drill string must be removed in order to replace the drill bit. As mentioned, this “trip” of the drill string is extremely time consuming and expensive to the driller. Likewise, since the regions of the cutting surface designed or tailored to be disposed on the trailing side of the insert relative to the direction of impact with the formation are generally less proficient at removing formation material, inserts oriented in such a manner may detrimentally reduce the cutting efficiency and ROP of the bit.
Accordingly, there remains a need in the art for a drill bit with cutting elements that will provide a relatively high rate of penetration and footage drilled, while at the same time, minimize the effects of wear and the tendency for breakage. Such bits would be particularly well received if the orientation and placement of the individual cutting elements accounted for the kinematics of the entire bit (i.e., rotation of the bit about the bit axis in conjunction with the rotation of the rolling cone cutters about their respective cone axes).